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FEM Modelling of the Plastic Flow Localization Induced in Metals by Surface Cracks

Marek P., Kurzydłowski K. J., Rożniatowski K.

Machine Dynamics Problems

2002

Vol. 26, No. 4

69-81

The aim of the present work is to provide a model for the influence of the surface films on the plasticity of metals. To this end Finite Element Method has been used to solve 2-dimensional plain strain problem. The effect of the film cracks on the deformation of the substrate for the specimens subjected to tensile straining has been studied using a number of models which different in the details of cracks geometry and origin as well as in the properties of both oxide film and metallic substrate. As far as the geometry is concerned, the following situations were analyzed: symmetric cracks on both sides of specimens - a) and a-symmetric crack on one side - b). For each of these two cases the crack depth has been varied in the range from 70 to 100% of the film thickness. As far as the cracks beginning is discussed the following two types of the cracks have been considered: * pre-existing cracks (existing in the film prior to the straining of the specimen), * fresh cracks (which develop during the straining). The model employed in the present study shows, that the macroscopic yielding of the specimens depends to some extent on the presence of the cracks in the surface film and is profoundly changed due to the presence of the residual stresses. The residual stresses due to the differences in the thermal expansion coefficients of the surface films and the substrates have been observed in the number of metals subjected to the annealing or surface treatment. The results reported here prove that these stresses deeply modify the flow stress of metals in the range of the micro-plasticity.

Influence of oxygen and humidity in atmospheres on friction and wear characteristics of TiN and TiC

Yang L., Yamamoto Y., Hashimoto M.

Applied Mechanics and Engineering

1999

Vol. 4, no spec.

195-200

The friction and wear characteristics of hot pressed TiN and TiC were examined in dry and wet atmospheres of oxygen and argon using a pin/disc type friction tester. In an atmosphere containing oxygen or humidity, for both TiN and TiC, a surface film composed mainly of titanium oxides and/or their hydrates were produced during runs. The formation of the surface films resulted in suppressing brittle fracture of the rubbing surfaces and reducing wear amount by a factor of 4 to 30. TiC has high ability to form TiO2 with low shear strength compared to TiN. In contrast, TiN is expected to keep a TiO2 film firmly through TiO formed on the TiN surface as the substrate of TiO2, since a lattice constant and interatomic distances of TiN are nearly equal to those of TiO. Therefore, a low friction of about 0.1 was attained with a combination of TiC pins and TiN disc.